In the area of polarization optics rotator is one basic element and it is essentially important because in a polarization-optical system polarized light and polarizing element are both characterized by their azimuths or orientation angles. The most important rotator is optical rotator or circular retarder. An element of a device whose Jones matrix (cf. R. C. Jones, J. Opt. Soc. Amer. 31, 1941, p. 488-493) R is of the form ##EQU1## is defined as an optical rotator or a circular retarder (cf. R. M. A. Azzam and N. M. Mashara, Ellipsometry and Polarized Light, North-Holland, N.Y. 1988), where .alpha. the rotation angle. The phenomenon described by Equation (1) is also known as optical activity. The state of polarization of a light beam is altered when it is reflected, scattered or propagated through a medium which exhibits either optical activity or birefringence.
In general, an element or a device that can rotate the vibration plane of a linearly polarized light a certain amount without affecting its other properties is called a polarization rotator. Half-wave plate rotator is the simplest polarization rotator, but it is widely employed. For a linearly polarized incident light whose vibration plane makes an angle .rho. related to the fast or slow axis of a half-wave plate, the vibration plane of the emergent beam from this half-wave plate will be rotated through an angle of 2.rho.. To adjust the rotation angle 2.rho., it is necessary to rotate the half-wave plate with respect to the input polarization plane. Requirement of mechanical adjustment of the rotation angle of the half-waveplate rotator restricts its application. This weakness is especially undesirable for some applications, for example, when a half-wave plate is followed by a polarization beamsplitter for making a variable ratio beamsplitter for linearly polarized monochromatic input. Because the ratio of the two emergent orthogonal beam irradiances from the beamsplitter are determined by the rotation angle of the half-wave plate, an electrically controllable inertialess adjustment of the rotation angle should be very desirable in this case.
Another kind of polarization rotator is Senarmont rotator. A Senarmont rotator consists of a retarder followed by a quarter-wave plate whose fast or slow axis is oriented with an angle of 45.degree. related to the the fast axis of the retarder. A Senarmont rotator will rotate the vibration plane of a linearly polarized light aligned to be parallel to the fast or slow axis of the quarter-wave plate through an angle equal to one-half the retardance of the retarder. By simply replacing this retarder with a polarization modulator or variable retarder, a continous polarization rotation can be accomplished. An application limitation for the Serotator is that the vibration plane of the incident light must be oriented parallel to the fast or slow axis of the quarter-wave plate. Moreover, as a polarization rotator it is only suitable for rotating a linearly polarized light.
Faraday rotator, which is made based on the Faraday effect, is a special kind of rotator and can be used as a polarization rotator, too. The rotation angle of a Faraday rotator is proportional to the so-called Verdet constant, the intensity of the magnetic field and the path length of light beam through the rotator. By varying the intensity of the magnetic field a continuous variation of the rotation angle is to be performed. The difference of the Faraday effect from the optical activity lies in the fact that when a linearly polarized light travels through a Faraday rotator and is reflected back along its own path, the Faraday rotation is cumulative, since its sense is dependent on the direction of light propagation, while in the same case the sense of a rotation due to optical activity is independent on the direction of description, thus the resultant rotation will be exactly cancelled out (cf. D. Clarke and J. F. Grainger, Polarized Light and Optical Measurement (Pergamon Press, Oxford, 1971 )).
From the viewpoint of applications the main weakness of the Faraday rotator is that the magneto-optic materials are generally not transparent in the visible regions and have thus fund more use only in the near-infrared and infrared regions (cf. J. M. Hammer, "Modulation and Switching of Light in Dielectric Waveguides," in Integrated Optics, T. Tamir, ed. (Springer-Verlag, Berlin, 1975), Chap. 4.). Even in these spectral regions, many materials with large Faraday rotations still strongly absorb light. The other disadvantages with Faraday rotators are the temperature- and frequency-dependence of the Verdet constant and the magnetic hysteresis effect. Compared with a polarization rotator, an optical rotator has much wider applicability range, because Equation (1) physically describes a rotation coordinate transformation exactly. Optical rotator can find applications in all the fields in which polarized light is used, especially in optical ellipsometry and polarimetry. An optical rotator can be used for rotating not only a linearly polarized light but also an elliptically polarized light. Furthermore, it can be used for rotation of the reference axis of a polarizing element or device such as polarizer, waveplate and polarization modulator. This potential application of optical rotator is of particular importance because it will enable inertialess and photoelectrically controllable change of the azimuths of polarizing elements or devices. On the other hand, unlike a polarization rotator, an optical rotator is rotation-coordinate-independent (this feature can be easily proved from Equation (1)). This advantage of the optical rotator will enable its convenient usage without any coordinate alignment restriction related to the polarized light or polarizing element that it associates with. However, the presently available opt rotators, which are plates being made of quarz crystal exhibiting circular birefringence (e.g. CVI Laser Corporation, Albuquerque, N. Mex. U.S.A.), can be used only for a special angle of rotation. The fixed angle of rotation restricts their applications very much.